1. Field of the Invention
The present invention relates to a switching power supply unit in which a switched output obtained by switching a DC input voltage is taken out in an output winding of a power converting transformer.
2. Description of the Related Art
Hitherto, various types of DC-DC converters have been proposed as a switching power supply unit and provided for practical use. Many of them are of a type in which a DC input voltage is switched with a switching operation by a switching circuit (inverter circuit) that is connected to a primary winding of a power converting transformer (transformer device), and the switched output (inverter output) is taken out in a secondary winding of the power converting transformer (transformer). The voltage appearing in the secondary winding in accordance with the switching operation of the switching circuit is rectified by a rectifier circuit, then is converted into a direct current by a smoothing circuit and outputted.
In a switching power supply unit of this kind, an output rectifier device such as an output rectifier diode is connected in series with a power transmission line in the rectifier circuit. Therefore, it is extremely effective for improving operative efficiency of the switching power supply unit to reduce loss in the output rectifier diode.
Reduction of loss in the output rectifier diode is effectively realized by using a diode having a small forward voltage drop. However, since the diode having a small forward voltage drop has also a low reverse withstand voltage, in order to use a diode having a small forward voltage drop as the output rectifier diode, it is particularly necessary to suppress the reverse voltage.
In a switching power supply unit of this kind, what to be most considered as such reverse voltage is a surge (spike) voltage caused by a parasitic element at the time of on/off operation of the switching circuit. It is to be noted that the surge voltage is applied as a reverse voltage to the output rectifier diode. Accordingly, various ideas have been executed in the past to suppress such surge voltage (refer to U.S. Pat. No. 4,864,479 and Japanese Patent Application Publication No. 2008-187801, for example).